I've read a few articles recently where 'air pruning roots' has been mentioned. Am I right in believing that this process is a positive and beneficial one for horticulturists in that it promotes root expansion and therefore a greater mechanism by which nutrients can be supplied to the plant resulting in quicker and/or bigger growth?

It sounds like its the underground version of pinching out a growing tip to encourage bushiness.

Are there any plants that we here are likely to encounter where this type of growing technique is detrimental?

I have noticed that some date stones that I germinated recently in vending machine cups had coiled tap roots where they were unable to exit through the drainage holes and subsequently air prune by themselves. It seems that although they were healthy, I could have maximised their growth this summer with a bit more consideration to their potting. I have now increased the drainage holes to 8mm each (x 5 per cup) from about 3mm to see if that works.

It can work Mike, but as you say not on everything. Some palms for instance will grow a new root if one is chopped, so you can end up with more roots. Others may branch so will end up with a better network of roots, but some, like sabal for instance, the root dies back to the growth point when cut. This will cause the constant loss of roots and will be detrimental to growth.

When I went to Kew with EPS back in 2008 I think. We were shown around the propagation areas. They had some Livistona in what I would describe as hedgehog tubs. These had cones all over the surface and looked like it was to direct roots out of holes at the tip of the cones, rather than allowing them to coil. Others of the same species were in standard tubs. Those in the standard tubs were were looking fine, but those in the hedgehog tubs were smaller, more weedy, and very sickly looking.

The (even shorter) short version:
- they only compared two species, so it is not conclusive (howwever they are at least trying to answer the question)
- in the study, air pruning did not increase growth (by promoting side roots for example)
- in the study, it retarded the growth of one species, but this appears to vary by species
- the best thing for the palms was to provide bigger (and presumably deeper) pots, this produced more roots, more leaves and a higher quality plant.

Unfortunately all my palms air prune themselves out of the bottom of any pot I put them in all too soon. Having seen the report above, I suspect I'm actually stunting them by not re-potting often enough.

No surprise that it retards growth a bit, as a plant in one has fewer roots than one in a normal pot. But that's not the point, the reason for using them is that they prevent coiled roots, which can be very damaging for plants later in their life - coiled roots make plants unstable and very liable to blow down in even light winds, and/or can strangle themselves and die.

Conifers wrote:No surprise that it retards growth a bit, as a plant in one has fewer roots than one in a normal pot. But that's not the point, the reason for using them is that they prevent coiled roots, which can be very damaging for plants later in their life - coiled roots make plants unstable and very liable to blow down in even light winds, and/or can strangle themselves and die.